Use of ion implantation to facilitate the discovery and characterization of ferromagnetic semiconductors

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Abstract
The discovery of epitaxially grown ferromagnetic, type III–V semiconductors (Ga,Mn)As (Tc=110 K) and (In,Mn)As (Tc=35 K) holds promise for developing semiconductor electronics that utilize the electron’s spin degree of freedom in addition to its charge. It has been theoretically predicted that some semiconducting systems could be ferromagnetic above room temperature, when optimally doped (p-GaN with 5% Mn). We report here on the use of ion implantation to incorporate magnetic ions into a variety of semiconducting substrates, thereby facilitating investigation of the nature of ferromagnetism in semiconducting systems that are difficult to grow with other methods. The magnetic ions, Mn, Fe, and Ni, were implanted into each of the epitaxially grown semiconductors GaN, GaP, and SiC to achieve volume concentrations between 1 and 5 at. %. The implanted samples were subsequently annealed at 700–1000 °C to recrystallize the samples and remove implant damage. The implanted samples were examined with both x-ray diffraction and transmission electron microscopy to characterize their microstructure and with superconducting quantum interference device (SQUID) to determine magnetic properties. In most cases, no secondary phases were found. The magnetic measurements [hysteresis, coercive fields, and differences between field-cooled (FC) and zero field-cooled (ZFC) magnetizations] indicate ferromagnetism up to room temperature for some samples that could not be attributed to superparamagnetism or any other magnetic phase. Particularly, p-GaP:C with high hole concentration, when doped by implantation with 3 at. % Mn, showed ferromagnetic behavior very close (Tc=250 K) to room temperature. In summary, we found that ferromagnetic behavior is very dependent on the concentration of the magnetic impurities for all samples and it is even more dramatically affected by the type and the concentration of the majority carriers, in qualitative agreement with the theory.